The present invention relates to a gas-filled discharge tube for use as transient protector, with at least two electrodes separated by a discharge gap and an insulating body which is joined to the electrodes and, jointly with them, forms a discharge chamber. The sealing of the tube occurs at atmospheric pressure and a suitable temperature, in a mixture of a light and a heavy gas suitable for the purpose, after which this gas mixture is pumped out or any other such step is taken until the gas pressure around the tube drops below atmospheric pressure and is suitably reduced to the order of magnitude of 3 kPa or less, while at the same time the temperature is adjusted in such a manner that the light gas enclosed in the tube, through diffusion or effusion, because of the pressure difference between the inside of the tube and its surroundings, exits through the tube walls, whereas the heavy gases can diffuse through the tube walls to an insignificant extent only. The gas diffusion brings about a reduction in the total gas pressure inside the tube and the process is interrupted when the desired pressure drop is obtained.
Gas filled discharge tubes are extensively used as protection against transient voltages in electronic equipment of various kinds, e.g. telephone equipment, computers, and safety systems. Discharge tubes for this special purpose consist, as a rule, of at least two electrodes which, with a suitable spacing between them, are joined to an insulating body, so as to form at least a discharge gap in a discharge chamber that is vacuum-tight at normal temperatures and which encloses a gas of a suitable kind at a suitable pressure. The insulating body is, as a rule, made of ceramics. Two electrode tubes are used most often. The tube can be connected between conducting points which can be exposed to transient voltages, or between such a point and the ground. These electrode tubes are also being used. A central electrode, as a rule, is connected to the ground, whereas the outer electrodes are connected to the points to be protected.
The electrode material of the tube, the size of the electrode gap and the type and pressure of the gas will determine the starting or striking voltage of the tube. The latter must be adjusted in such a way that the tube will not ignite at the normally applied voltages. However, if voltages originate which could harm the equipment, the tube will ignite and cause the voltage to drop across the tube and thus across the protected equipment, thereby preventing the occurrence of damage.
Since the discharges occurring between the electrodes generate a displacement of materials between same, there is a certain risk of disturbances in the form, e.g., of short circuits in the electrode gap. For that reason, gaps between the electrodes are rarely very small. The gaps mostly used are of the order of magnitude of 0.4-0.5 mm. As a filler gas, argon is frequently used, possibly with an addition of about 10% of hydrogen. Sometimes krypton and xenon are used instead of argon. In connection with copper electrodes and an electrode gap of about 0.5 mm, a gas pressure is frequently selected with measures about 10 kPa at normal room temperature. This will result in a frequently desirable starting voltage of about 350 V. If, for any reason, it is desired to manufacture a tube for higher gas pressure, the electrode gap should be reduced accordingly, since the product of gap and pressure must not change if the starting voltage is to remain unchanged. As already stated above, gap disturbances can be a problem in such a case.
As mentioned before, an insulating body of ceramics, designed as hollow cylinder, is used most frequently. The tube is usually manufactured in such a way that the end surfaces of the ceramic unit are metalized, often by means of a coat of molybdenum-manganese and an overcoat of nickel. The electrodes can then be soldered to this metalized layer. If the electrodes are of copper, e.g. a silver-copper eutectic can be used as suitable soldering material at about 800.degree. C. Other electrode material may require a different kind of soldering metals. As a rule, the soldering, together with the rest of the process, follows either one of the two methods described below.
In one case a ring of soldering material is placed on the portion of the electrode surface to be soldered to one metalized end surface of the ceramic tube. The ceramic tube is placed on the solder ring, while a new solder ring is placed on the other metalized end surface of the ceramic tube, and the second electrode is placed on that solder ring. This electrode has been provided with a traversing narrow copper tube, to form an open channel into the internal volume of the tube. The soldering is done most often in a belt furnace with reducing gas, usually hydrogen or a mixture of hydrogen and nitrogen. The temperature depends on the soldering material, with the silver-copper eutectic, this is, as mentioned, about 800.degree. C. The soldering is followed by vacuum-pumping at about 400.degree. C., and by a refilling or replenishing with the desired gas up to the desired pressure. Pumping and refilling are frequently performed manually. However, semi-automatic methods or devices are, occasionally, also used. Vacuum-pumping and refilling occur through the tube, the so-called pump out or exhaust tube, which one of the electrodes has been provided with. At times, a small portion of the refilling gas is replaced by tritium, a radioactive isotope of hydrogen, which has a certain stabilizing effect on the starting voltage of the tube. After vacuum-pumping and gas refilling have been completed, the copper tube is nipped off near the electrode. This nipping off, as a result of cold diffusion, will cause a vacuum-tight joint. The manufacturing process concludes with an electrical stabilizing treatment prior to the final test.
The second method calls for stacking of the tube parts in the same manner as above. However, in this case no exhaust tube is used. The stacked tube parts are placed in suitable numbers on a plate of suitable material, and a number of these plates are placed, jointly, into a furnace. The furnace is pumped down to a vacuum of about 0.01 Pa at a temperature slightly below the melting point of the soldering material. Since the tube parts are stacked loosely, there will also be a vacuum in the internal spacing of the stacks. Subsequently, at the same temperature, refilling gas is fed into the furnace and thus also into the stacked tubes. The temperature is then raised, and thus, the tubes are soldered together within the gaseous atmosphere. As in the previous case, after cooling, an electrical stabilizing treatment is applied to the final test.
Also in the case of tubes manufactured according to the second method, it is sometimes desired to mix a small amount of tritium with the filling gas. However, since the gas in the furnace is, after cooling, ventilated after each pumping turn or cycle, it is inconceivable to mix tritium with the filling gas. Instead, a diffusion process, as disclosed in Swedish Pat. No. 375,201, is frequently used after the completed pumping process.
Variants of these two manufacturing methods are sometimes used, but the feature that all of them have in common is that they are rather cumbersome, require much energy and manufacturing equipment, and are not suited for automatic in-line production.
A more in-line oriented production method has been proposed in Swedish Patent Application No. 7,910,359-4. This manufacturing method is based, in part, on the property of the gases to equalize, through diffusion, any partial pressure differences within the limits of volume and, in part, on the varying capacity of the gases to penetrate, e.g. glass and ceramics, through diffusion, and in many cases also through effusion. This penetration varies considerably among the gases. The difference may be as high as several times the 10th power between a gas with a small molecular diameter and low density, e.g. hydrogen and helium, and a gas with a large molecular diameter and high density, e.g. argon, krypton, and xenon. For purposes of this invention, gas with a small molecular diameter and low density shall be designated as a light gas, whereas a gas with a large molecular diameter and high density shall be called a heavy gas.
The method signifies that the tube parts are stacked as in other manufacturing methods described above. No exhaust tubes are used. After the stacking, the tube stack is placed in a furnace with a gas mixture that is substantially at atmospheric pressure and consists of suitable light and heavy gas, e.g. hydrogen and argon. This gas mixture will envelop the tube and also enter it and replace the air therein. This occurs at a temperature that is gradually increased and, after a suitable time, will reach the sealing temperature. After the sealing, the temperature is lowered to slightly below the sealing temperature and the sealing gas is replaced by heavy gas only, e.g. argon, substantially at atmospheric pressure. Since the partial pressure of the hydrogen is now higher inside the tube than outside thereof, a portion of the hydrogen enclosed in the tube will, at the prevailing temperature, diffuse through the tube wall. The partial pressure of the argon is higher outside the tube than inside thereof, but argon cannot penetrate the tube wall to any significant extent. When the desired argon/hydrogen ratio and the total pressure in the tube has been reached, the diffusion is interrupted and the tube is cooled to a suitable temperature.
The process can of course be performed in a conventional stationary furnace with gas flow facilities and temperature control. But the method is also suited for in-line production. A belt-fed furnace with sections for heating/sealing, respectively diffusion and subsequent cooling can be used to perform the method. Since the entire process takes place mainly at atmospheric pressure, only simple partition walls with a passage opening for the conveyer belt and the tubes or plates with the tubes placed thereon are needed to adequately separate the various sections. As in the other above-described prior art methods, the cooling is followed by an electric stabilization treatment before final testing. Also in this case tritiation may be applied by means of tritium diffusion.
This method is simple and suited for in-line production. However, it makes special demands on the diffusion properties of the material used in the tube parts, if the diffusion time is to be kept within reasonable limits. In the present situation, this requires additional cost for the tube parts, as compared to the cost of using more conventional material.